In an expander cycle, the fuel is heated before it is combusted,
usually with waste heat from the main combustion chamber. As the
liquid fuel passes through coolant passages in the walls of the
combustion chamber, it undergoes a phase change into a
gaseous state. The fuel in the gaseous state expands through a
turbine using the pressure differential from the supply pressure to
the ambient exhaust pressure to initiate turbopump rotation. This
can provide a bootstrap starting capability as is used on the Pratt &
WhitneyRL10 engine. This
bootstrap power is used to drive turbines that drive the fuel and oxidizer pumps
increasing the propellant pressures and flows to the rocket engine
thrust chamber. After leaving the turbine(s), the fuel is then
injected with the oxidizer into the combustion chamber and burned
to produce thrust for the
vehicle.

Because of the necessary phase change, the expander cycle is
thrust limited by the square-cube rule. As the size of a
bell-shaped nozzle increases with increasing thrust, the nozzle
surface area (from which heat can be extracted to expand the fuel)
increases as the square of the radius. However, the volume of fuel
that must be heated increases as the cube of the radius. Thus there
exists a maximum engine size of approximately 300 kN of thrust beyond which there is no
longer enough nozzle area to heat enough fuel to drive the turbines
and hence the fuel pumps. Higher thrust levels can be achieved
using a bypass expander cycle where a portion of the fuel bypasses
the turbine and or thrust chamber cooling passages and goes
directly to the main chamber injector. Aerospike engines do not suffer from the same
limitations because the linear shape of the engine is not subject
to the square-cube law. As the width of the engine increases, both
the volume of fuel to be heated and the available thermal energy
increase linearly, allowing arbitrarily wide engines to be
constructed. All expander cycle engines need to use a cryogenic fuel
such as hydrogen, methane, or propane that easily reach their boiling points.

Some expander cycle engine may use a gas generator of some kind to start the
turbine and run the engine until the heat input from the thrust
chamber and nozzle skirt increases as the chamber pressure builds
up.

In an open cycle, or "bleed" expander cycle, only some
of the fuel is heated to drive the turbines, which is then vented
to atmosphere to increase turbine efficiency. While this increases
power output, the dumped fuel leads to a decrease in propellant
efficiency (lower engine specific impulse). A closed cycle
expander engine sends the turbine exhaust to the combustion chamber
(see image at right.)

Contents

Expander Bleed Cycle
(Open Cycle)

This operational cycle is a modification of the traditional
expander cycle. In the bleed (or open) cycle, instead of routing
heated propellant through the turbine and sending it back to be
combusted, only a small portion of the propellant is used to drive
the turbine and is then bled off, being vented overboard without
going through the combustion chamber. Bleeding off the turbine
exhaust allows for a higher turbopump output by maximizing the
pressure drop through the turbine. This leads to higher engine
thrust at the sacrifice of some efficiency loss due to essentially
wasting the bled propellant. However, in some cases, such as the
Japanese LE-5A/B, the performance
gains can take precedence over absolute efficiency.

Advantages

The expander cycle has a number of advantages over other
designs:

Low temperature. The advantage is that after they have turned
gaseous, the fuels are usually near room temperature, and do very
little or no damage to the turbine, allowing the engine to be
reusable. In contrast Gas-generator or Staged combustion
engines operate their turbines at high temperature.

Tolerance. During the development of the RL10 engineers were worried that insulation foam
mounted on the inside of the tank might break off and damage the
engine. They tested this by putting loose foam in a fuel tank and
running it through the engine. The RL10 chewed it up without
problems or noticeable degradation in performance. Conventional
gas-generators are in practice miniature rocket engines, with all
the complexity that implies. Blocking even a small part of a gas
generator can lead to a hot spot, which can cause violent loss of
the engine. Using the engine bell as a 'gas generator' also makes
it very tolerant of fuel contamination because of the wider fuel
flow channels used.

Inherent safety. Because a bell-type expander-cycle engine is
thrust limited, it can easily be designed to withstand its maximum
thrust conditions. In other engine types, a stuck fuel valve or
similar problem can lead to engine thrust spiraling out of control
due to unintended feedback systems. Other engine types require
complex mechanical or electronic controllers to ensure this does
not happen. Expander cycles are by design incapable of
malfunctioning that way.